1. Field of the Invention
The present invention relates to a method to measure the brake force for railroad vehicles and it relates also to a portable device to measure the brake force for railroad vehicles according to the above-mentioned method. More specifically the present invention is related to a method to measure the brake force for railroad vehicles without replacing the brake shoe, brake pad or any other component of the braking system and to display the measured value straight in force units.
2. Brief Description of the Prior Art
Railroad vehicles are generally equipped with an air braking system, which typically for freight railcars and locomotives, operates a brake shoe assembly that in turn applies the brake force to the wheel tread. Some passenger coaches use a different braking system the main components of which are a brake disc and brake pads with the pads being pressed against the disc when the brake is applied.
Government regulations related to railroad safety operation require braking system testing for all new equipment, before they are released for service, and for all in-service equipment. These testing address mainly the performance of the braking systems, the wear aspects and the brake force. Maintenance procedures in force at every railroad administration also call for periodic testing of the braking systems. The American Association of Railroads (AAR) require certain minimum acceptable brake force levels for certain vehicle weights to insure proper braking of railway equipment. The AAR also require specific standards for measuring the rake force.
All the measuring devices currently in use to measure the brake force for systems using a brake shoe pressing against the wheel tread require the removal of the actual brake shoe and its replacement with a special brake force-measuring shoe.
The oldest known device (still being used today) to measure the brake force is the so-called simulated brake shoe. This device looks like a conventional brake shoe but it is made of steel (instead of cast iron or composition) and has strain gauges bonded on it. To measure the brake force, the actual brake shoe must be replaced with this simulated brake shoe, which is large and heavy; the signal generated and displayed is then processed by qualified personnel to determine the brake force magnitude. The measurement errors, the largest when compared with errors from any other known brake force measuring device, are related to the friction in the rigging and to the positioning of the simulated brake shoe.
Another known and widespread device is the single contact point wheel/compression load cell (also known under the name xe2x80x9cJim Shoexe2x80x9d) which uses one xe2x80x9cready madexe2x80x9d load cell mechanically attached to an adapter and connected through a switching box to an indicator. This device is unstable, hazardous and consequently generates another class of measurement errors. When the brake force is applied, the compression load cell button slides over the wheel tread surface until equilibrium is reached. At this random equilibrium location, the output signal of the measuring system is altered by the effect of bending due to the non-radial position of the compression load cell, leading to nonxe2x80x94repeatable brake force test results. Furthermore, the single point contact unstable condition is magnified when the brake forces are fast applied (for instance in case of an emergency braking), when relatively high braking forces are required and when wider shoe rotations are allowed by the brake specific design.
One other known brake force measuring device (called by its designers xe2x80x9crailcar brake testerxe2x80x9d) is described in U.S. Pat. No. 5,038,605. Because of the multiple contact points feature of this device, the sliding effect is virtually eliminated. However, its very complex design (passage ways for the electric wires, retainer being held in place by threaded members, the general design of the cell body with bent endings, an asymmetrical design related to the mechanical attachment of the electrical connector) leads to errors related to the nonxe2x80x94linearity and nonxe2x80x94repeatability of the measurements. Also, the friction between the retainer and the shoe head, which cannot be controlled from one test to the next and even during the same test, will generate another class of errors.
As it can be seen, all the measuring devices currently in use to measure the brake force for systems using a brake shoe pressing against the wheel tread, suffer from a variety of shortcomings. All require the removal of the actual brake shoe and its replacement with a special brake force-measuring shoe. These measuring shoes are bulky, heavy, difficult to transport and therefore inconvenient to use. Also, as it was mentioned before, for most devices the displayed value must be further processed by qualified personnel to determine the brake force magnitude expressed in specific units (lb, N, Kgf). Furthermore, because the replacement of the actual brake shoe with a measuring shoe alters the friction in the rigging assembly, the measured brake force is prone to errors. Other shortcomings, mentioned above, were related to the non-linearity, non-repeatability and relatively poor accuracy of the measurements.
Because of the complexity of the existing brake force measurement methods and sensor configurations, as a recommended railway practice, the brake cylinder pressure is frequently measured in order to obtain an indirect brake force assessment. This practice is not a measurement per se; it only makes it possible to estimate the brake force magnitude by computing. As such, it cannot be compared with a brake force-measuring device. It should be noted, however, that the accuracy of this estimation is poor.
The purpose of the present invention is to provide a new, simpler and more accurate method to measure the brake force for railroad vehicles and which can be used for any type of railcar braking systems. The present invention eliminates the cumbersome task of replacing the actual brake shoe with a specialized measuring shoe. Instead, according to this method, a clearance is opened between the main braking components (brake shoe and wheel tread or brake pads and brake disc), clearance, which is large enough to allow the insertion of a special sensor between these components. The main part of this sensor is a prismatic steel plate with strain gauges bonded on two opposite sides. When the brake is applied, the sensor is compressed between the braking surfaces. Unlike the configuration used by existing brake force testers (single or multiple contact points with the braking surface), the sensor according to the invention comes in contact with the braking surfaces (brake shoe/wheel tread or brake pad/brake disc) through the intermediary of four drill rods. The rods are affixed to the sensor body, two facing the brake shoe and the other two facing the wheel tread. Thus, the mechanical contact between the four rods and the sensor body is over four lines. This configuration virtually eliminates a class of errors related to the non-repeatability of brake force measurements The rods are positioned in such a way (the two on the wheel side near the center of the sensor and the two on the brake shoe side near opposite edges of the sensor) that when the brake is applied the sensor body will be submitted to a bending moment. The signal generated by the strain gauges is processed and displayed on an indicator calibrated in force units. To insure its adaptability to any wheel diameter or wear of the brake components, to improve the consistency of the measurements and to prevent the brake shoe surface deterioration, the sensor uses two steel wings (steel plates of rectangular shape). These wings are elastically seated on the two rods affixed on the brake shoe side of the sensor (for braking systems using a brake shoe) and they are kept in place by rubber membranes. During measurements, the sensor is kept in place by magnets elastically connected to the sensor body.
It is, therefore, one of the primary objects of the present invention to provide a method and a device (the main part of which is a sensor) to measure the brake force for railroad vehicles, which do not require the removal of the brake shoe and its replacement with a special instrumented brake shoe.
Another object of the present invention is to provide a sensor, which is small enough to allow its insertion between the main braking components (brake shoe/wheel tread or brake pad/brake disc).
Still another object of the present invention is to provide a sensor, to be used for measuring the brake force, whose performance is not dependent on the wear condition of the brake shoe.
Yet another object of the present invention is to provide a sensor, to be used as above, whose performance is not dependent on the brake shoe material (cast iron or composition).
A further object of the present invention is to provide a sensor which, when used to measure the brake force, does not alter the friction in the rigging.
An additional object of the present invention is to provide a sensor whose performance, when used as above, does not depend on the rail vehicle wheel diameter.
Another object of the present invention is to provide a sensor, to measure the brake force, which can be used for any position of the brake shoe relative to the wheel (or brake pad relative to the brake disc).
Still yet another object of the present invention is to provide a sensor having means, which allow to position it and to keep it in place during measurements.
A still further object of the present invention is to provide a measuring device (sensor) for the brake force whose indication does not change when repeated measurements on the same brake configuration are done (high degree of repeatability) and this feature does not depend on the wheel diameter, brake shoe material (cast iron or composition), brake shoe positioning relative to the wheel, brake shoe wear and wheel tread wear.